# std::fill

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt, class T > void fill( ForwardIt first, ForwardIt last, const T& value ); |  | (constexpr since C++20) (until C++26)
template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > constexpr void fill( ForwardIt first, ForwardIt last, const T& value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt, class T > void fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > void fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++26)

1) Assigns the given value to all elements in the range `[`first`,` last`)`.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If value is not writable to first, the program is ill-formed.

### Parameters

first, last | - | the range of elements to modify
value | - | the value to be assigned
policy | - | the execution policy to use
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Complexity

Exactly std::distance(first, last) assignments.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

fill
    template<class ForwardIt,
             class T = typename std::iterator_traits<ForwardIt>::value_type>
    void fill(ForwardIt first, ForwardIt last, const T& value)
    {
        for (; first != last; ++first)
            *first = value;
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

Run this code

    #include <algorithm>
    #include <complex>
    #include <iostream>
    #include <vector>

    void println(const auto& seq)
    {
        for (const auto& e : seq)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8};
        println(v);

        // set all of the elements to 8
        std::fill(v.begin(), v.end(), 8);
        println(v);

        std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
        println(nums);
        #ifdef __cpp_lib_algorithm_default_value_type
            std::fill(nums.begin(), nums.end(), {4, 2});
        #else
            std::fill(nums.begin(), nums.end(), std::complex<double>{4, 2});
        #endif
        println(nums);
    }

Output:

    0 1 2 3 4 5 6 7 8
    8 8 8 8 8 8 8 8 8
    (1,3) (2,2) (4,8)
    (4,2) (4,2) (4,2)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be CopyAssignable, but `T` is not always writable to `ForwardIt` | required to be writable instead

### See also

fill_n | copy-assigns the given value to N elements in a range (function template)
copycopy_if (C++11) | copies a range of elements to a new location (function template)
generate | assigns the results of successive function calls to every element in a range (function template)
transform | applies a function to a range of elements, storing results in a destination range (function template)
ranges::fill (C++20) | assigns a range of elements a certain value (algorithm function object)

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